R1:a0d4b6da4b60

Support (but do not actually enable) a maximum file size limit for Git repositories

Summary:
Depends on D19816. Ref T13216. See PHI908. See PHI750. In a few cases, users have pushed multi-gigabyte files full of various things that probably shouldn't be version controlled. This tends to create various headaches.

Add support for limiting the maximum size of any object. Specifically, we:

- list all the objects each commit touches;
- check their size after the commit applies;
- if it's over the limit…